feat(db): add Db handle with pool connect and readiness ping

crates/db/tests/ping.rs

@@ -0,0 +1,10 @@
+use db::Db;
+use sqlx::PgPool;
+
+#[sqlx::test]
+async fn ping_succeeds(pool: PgPool) {
+    let db = Db::from_pool(pool);
+    db.ping()
+        .await
+        .expect("ping should succeed against a live database");
+}